FIG. 18 shows the outline of a sheet forming machine for forming a sheet of a thermoplastic resin.
In FIG. 18, the numeral 11 denotes an extruder, 12 a die, 13 to 15 metallic rolls, and 16 a cutter.
As shown in this drawing, a melt resin 17 extruded from the die 12 of the extruder 11 is guided to the plurality of metallic rolls 13 to 15. Both surfaces of the melt resin 17 are pinched and cooled by the surfaces of the rolls, whereby the melt resin 17 is formed into a thin sheet 18. Then, the thin sheet 18 is cut to predetermined lengths by the cutter 16, or if desired, taken up by a winding device (not shown).
It is well known that when the melt resin 17 from the die 12 is pinched by the above resin forming machine into the thin sheet 18, sheet formation takes place while a bank is occurring at the nip between the rolls.
With earlier technologies, when the thin sheet 18 having a relatively large thickness (say, 300 .mu.m or more) is to be obtained, it is relatively easy to control the bank to make it uniform widthwise. A thick melt resin has some cushioning effect. Even if the bank is slightly nonuniform widthwise, therefore, the thick melt resin can be pinched over its entire width by the surfaces of the rolls nearly uniformly. As a result, a smooth sheet can be formed.
When the thin sheet 18 having a very small thickness of, say, about 200 .mu.m is to be formed by the rigid metallic rolls 13 and 14, on the other hand, the cushioning properties of the melt resin 17 are low. Thus, those parts of the melt resin 17 which have a bank are pressed against the surfaces of the metallic rolls, while bank-free parts are not pressed against the surfaces of the metallic rolls. Consequently, the sheet surface becomes uneven partially in the width direction.
The earlier technologies, therefore, have proposed that the surface of one of the metallic rolls be formed of an elastic material such as rubber so that the elastic force of the elastic material prevents a bank mark from occurring during sheet formation.
However, the surface of the elastic material such as rubber cannot be made a mirror surface similar to that of the metallic roll, thus causing unsatisfactory surface smoothness to the thin sheet.
Another problem is that a transparent sheet which must have mirror surfaces as both surfaces cannot be formed.
An additional proposal has been made of one of the metallic rolls which is a mirror finished roll covered with a metal tube outside of the elastic material (see Japanese Laid-Open Patent Publication Nos. 124425/90 and 100960/95).
The above-described proposal is intended to obtain a thin resin sheet having excellent surface smoothness. For this purpose, the melt resin to be formed into a thin sheet is pressed under the elastic force of the elastic material layer (e.g. rubber), disposed inside of the metallic tube, such that the elasticity accommodates the uneven thickness of the melt resin.